1. Field of the Invention
This invention relates to a method of dubbing compressed digital video signals adopted for an apparatus for efficient coding and subsequent recording/playback of the video signals.
2. Description of the Related Art
Digital VTRs of various digital formats, such as D1 mode, D2 mode and D3 mode, have been commercialized for broadcasting use. For practical digital VTRs for private purposes, also, various kinds of formats have been proposed.
For example, a digital VTR employing efficient coding due to discrete cosine transform (DCT) and variable length coding (VLC) is provided with a recording system and a playback system as shown in FIG. 1.
In the recording system of the digital VTR shown in FIG. 1, the following processing is implemented. An input analog-component video signal (Y, R-Y, B-Y) is converted into a digital-component video signal by an A/D converter 1, and is then divided into blocks of data, each block having a unit of 8.times.8, that is, 8 samples in the horizontal direction and 8 lines in the vertical direction, using a frame memory in a blocking circuit 2. The data is shuffled and Y/C multiplexed. The data of an 8.times.8 unit is processed with DCT by a DCT circuit 3, whereby the data of a time amplitude domain is converted into data of a frequency domain. In addition, the data processed with DCT is re-quantized by an encoder 4, and is processed with VLC such as two-dimensional Huffman coding, so as to be compressed. The VLC by the encoder 4 is so controlled as to have a fixed length for each buffering unit constituted by a predetermined number of DCT blocks, for example, 30 DCT blocks. The data processed with DCT is framed to have a product coding structure of error correction coding (ECC) by a framing circuit 5, and is provided with a parity by a parity generating circuit 6. The data is then channel-coded by a channel encoder 7 into a form suitable for magnetic recording and playback, and is recorded in a magnetic tape, not shown, as serial recording data.
On the other hand, in the playback system, the following processing is implemented. A playback data played back from a magnetic tape is processed with data detection and serial/parallel conversion by a channel decoder 8, and with time-base correction by a time-base correction (TBC) circuit 9. The playback data is then processed with error correction by an error correction (ECC) circuit 10, and is decomposed into a word unit of VLC by a de-framing circuit 11. The data is decoded and inversely quantized by a decoder 12, and is processed with inverse DCT by an inverse DCT circuit 13, so as to be data of an 8.times.8 unit. The data is processed with de-shuffling, Y/C separation and data interpolation by a de-blocking circuit 14, and is converted into an analog signal by a D/A converter 15, so as to be outputted as an analog-component video signal.
As this method of dubbing video signals in the digital VTR, the first to sixth methods indicated by (i) to (vi) in FIG. 2 can be considered.
With the first method (i), data which has been channel-decoded by the channel decoder 8 on a playback unit side is entered in the channel encoder 7 on a recording unit side via a digital interface 21, thus carrying out digital dubbing.
With the second method (ii), data which has been processed with time-base correction by the time-base correction circuit 9 on the playback unit side is entered in the channel encoder 7 on the recording unit side via a digital interface 22, thus carrying out digital dubbing.
With the third method (iii), data which has been processed with error correction by the ECC circuit 10 on the playback unit side is entered in the parity generating circuit 5 on the recording unit side via a digital interface 23, thus carrying out digital dubbing.
With the fourth method (iv), data which has been de-framed by the de-framing circuit 11 on the playback unit side is entered in the framing circuit 5 on the recording unit side via a digital interface 24, thus carrying out digital dubbing.
With the fifth method (v), a digital-component video signal produced by the de-blocking circuit 14 on the playback unit side is entered in the blocking circuit 2 on the recording unit side via a digital interface 25, thus carrying out digital dubbing.
With the sixth method (vi), an analog-component video signal produced by the D/A converter on the playback unit side is entered in the A/D converter 1 on the recording unit side, thus carrying out conventional analog dubbing.
However, with the first method (i), among the first to sixth methods (i) to (vi) considered to be the methods of dubbing video signals in the digital VTR, the error correction by the error correction circuit 10 is not implemented on the playback unit side. Therefore, errors in playback are accumulated by repeated dubbing. In addition, since a part having no playback signal is copied as it is at the time of variable-speed playback, a problem remains that a normal playback picture cannot be produced by playing back a copied tape.
With the second method (ii), since the time-base correction is implemented by the TBC circuit 9 on the playback unit side, a playback signal of the same data format as in normal playback can be produced even in the variable-speed playback. However, without the error correction on the playback unit side, the problem of propagation of errors due to repeated dubbing cannot be solved.
With the fifth method (v), the high-speed digital interface 25 with a high transmission rate of 216 Mbps is needed for dubbing a non-compressed digital-component video signal. In addition, there is a risk of deterioration of picture quality due to DCT.
Furthermore, with the sixth method (vi), since analog dubbing for dubbing an analog-component video signal is carried out, a problem remains of deterioration of picture quality due to dubbing.